When your blog is loaded, all links of this class .more-link (standard wordpress css class for "Read more..."
links) are modified to no longer send the user to that post's
single page display when clicked. Instead, the WordPress database is queried for that specific post, and all content
after the <!--more--> tag (i.e. only what you need) is returned.
The new content is then immediately displayed to the user with light animation (You can disable animation in plugin options),
inline with the opening content.

AJAX-read-more uses the jQuery framework already included with WordPress installations.

This plugin used GET ajax action (not POST, like other plugins), wordpress "template_redirect" action.

It's compatible with cache plugins (like Hyper Cache Extended).

AJAX-read-more: design of progress indicator and other behaviors - by CSS (example - in css/all.css)